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| United States Patent |
6,036,321
|
|
Wright
, et al.
|
March 14, 2000
|
Crystal isolation housing
Abstract
The crystal isolation system of the present invention protects a laser
crystal from environmental factors such as humidity and dust. A crystal is
placed in a housing which has two ports in it, and flexible tubes are
placed in each port. Cap pieces cover the ends of the tubes and prevent
deleterious environmental factors such as dust and moisture from entering
the isolation system while allowing laser light to enter the system. A
desiccator attached to the system removes moisture from the air inside the
system, and an led and detector apparatus are is used to help indicate the
remaining useful life of the desiccant in the desiccator.
| Inventors:
|
Wright; David (Redwood City, CA);
Sheng; Shinan S. (Saratoga, CA);
Reeder; Dennis (South Jordan, UT)
|
| Assignee:
|
Spectra Physics Lasers, Inc. (Mountain View, CA)
|
| Appl. No.:
|
857656 |
| Filed:
|
May 16, 1997 |
| Current U.S. Class: |
359/513; 372/39 |
| Intern'l Class: |
G02B 001/10; H01S 003/07 |
| Field of Search: |
372/41,66,39
359/513
|
References Cited
U.S. Patent Documents
| 3621273 | Nov., 1971 | Rorden | 307/88.
|
| 4024466 | May., 1977 | Cremosnik | 331/94.
|
| 4968121 | Nov., 1990 | Bruesselbach et al. | 350/354.
|
| 5430756 | Jul., 1995 | Hanibara | 372/108.
|
| 5497268 | Mar., 1996 | Tang | 359/513.
|
| 5539765 | Jul., 1996 | Sibbett et al. | 372/92.
|
| 5548606 | Aug., 1996 | Senn et al. | 372/41.
|
| 5563899 | Oct., 1996 | Meissner et al. | 372/39.
|
| 5606453 | Feb., 1997 | Walling et al. | 372/21.
|
Other References
Patent Abstracts of Japan, vol. 014, No. 411 (E-0973) Sep. 5, 2990 (JP 02
156583 A).
Patent Abstracts of Japan, col. 018, No. 174 (E-1530), Mar. 24, 1994. (JP
05 343812 A).
|
Primary Examiner: Davie; James W.
Attorney, Agent or Firm: Wilson Sonsini Goodrich & Rosati
Claims
What is claimed is:
1. An environmental isolation housing, comprising:
a crystal having a first face and a second face, and said crystal having a
first composition;
a first cap piece with a first side and a second side, said first side of
said first cap piece being substantially the same shape as said first face
of said crystal and positioned in contact with said first face of said
crystal such that atomic bonds are formed between said first side of said
first cap piece and said first face of said crystal, and said first cap
piece having a second composition, said second composition being
substantially distinct in atomic structure from said first composition;
and
a second cap piece with a first side and a second side, said first side of
said second cap piece being substantially the same shape as said second
face of said crystal and positioned in contact with said second face of
said crystal such that atomic bonds are formed between said first side of
said second cap piece and said second face of said crystal, and said
second cap piece including having the second composition, wherein said
first and second cap pieces cover said ends of said crystal to provide
environmental isolation of said crystal.
2. The environmental isolation housing of claim 1, wherein said first cap
piece is transparent.
3. The environmental isolation housing of claim 1, wherein said first cap
piece is at least partially reflective.
4. The environmental isolation housing of claim 2, wherein said second side
of said first cap piece has an antireflection coating on it.
5. The environmental isolation housing of claim 2, wherein said second side
of said first cap piece and said second side of said second cap piece both
have antireflection coatings.
6. The environmental isolation housing of claim 1, wherein said first cap
piece is made of F-Series Schott glass.
7. The environmental isolation housing of claim 1, wherein said first cap
piece has an index of refraction within 0.01 or less of an index of
refraction of said crystal.
8. The environmental isolation housing of claim 1, wherein said first
composition includes Lithium Tri-Borate.
9. The environmental isolation housing of claim 1, wherein said crystal
having a first material reacting with water vapor, and said first cap
piece and said second piece having a second material not reacting with
water vapor.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an environmental isolation housing, and
more particularly to an environmental isolation housing for a crystal used
in a laser system.
2. Description of the Related Art
Crystals used in laser light applications may have some of their critical
parameters altered by exposure to environmental factors. For example,
Lithium Tri-Borate (LBO) is hydroscopic, and chemically re-acts with water
vapor. Typically, when an LBO crystal is used in laser light applications,
two opposing faces of the crystal will be polished optically flat, and
anti-reflection coated, so that a laser beam will pass through the crystal
with very small loss.
The anti-reflection coating can provide a barrier to the water vapor.
However, the thermal expansion coefficients of LBO are very high and, at
present, no available antireflection coating materials are similar, so
very large mechanical stresses occur at the LBO-to-coating interface over
the typical 25.degree. C. to 160.degree. C. operating range. At the edges
of the coating, and at any tiny defect sites, water molecules may react
with the LBO and destroy the bond between the LBO and the coating. This
mechanism may damage or destroy the entire coating within a few weeks in a
high humidity environment. Although moisture may also be absorbed through
the unpolished sides of the crystal, it is absorbed very slowly and
typically only causes damage at a greatly reduced rate compared to the
damage cause by moisture absorption at the crystal face.
Other environmental factors in the atmosphere can also cause problems for
crystals used in lasers. For example, dust particles and some vapors react
with laser light in such a manner that they may move along the light beam
toward an optical surfaces. Once they are attached to an optical surface
they may scatter the light causing loss of power. Also, these contaminant
can absorb the light and cause crystal heating that distorts the laser
beam wavefront or possibly damages the surface of the crystal via
thermally induced stresses.
Typically, optical elements used with laser light have been protected from
dust and vapor contamination by sealing the optical elements in a very
clean assembly or by purging the assembly with a high-purity gas, usually
air or nitrogen.
Complete sealing, using clean room techniques, is satisfactory if no field
service or change of the optics is anticipated. A purge gas system allows
the optics or other components to be changed in the field. However, it
does not protect the optics during shipment or at times when the purge gas
system is not operating unless the gas pumping and purification system is
a sealed part of the assembly.
Additionally, alignment of the laser optical components is usually very
critical, often requiring mechanical alignment and stability within
0.0005.degree.. Complete sealing has the complication that altitude or
other atmospheric pressure changes produce substantial forces on the
mechanics of the optical system which can misalign them. Very stiff
structures or symmetrical designs can be used to counter the pressure
changes, but sometimes more deliberate pressure equalizing mechanisms are
needed.
Precise alignment of the optical components of a laser is also complicated
by the sealing system used. The seals must have controlled flexibility or
be adjustable using only small forces.
There is a need for crystal isolation methods which consider all the
chemical, optical, and precise mechanical requirements of lasers outlined
above.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a system which isolates a
crystal from the environment while allowing the crystal to be aligned to a
laser beam or as part of a laser system.
It is a further object of the invention to provide a system which can
detect the useful age of an atmospheric conditioning system such as the
remaining absorptive power of the desiccant in a desiccator.
In one embodiment of the invention the crystal is placed in a housing which
has two ports to allow a laser beam to enter and exit the housing. Cap
pieces are placed over the input ports in order to seal off the housing
from the environment. A desiccator or other air treatment device is then
attached to the system to remove unwanted elements from the atmosphere
inside the system and those that may leak in over time. A light source is
set up to direct light at the contents of the desiccator or air treatment
device. Light is reflected from or transmitted by the contents and
received by a detector. The amount of light received by the detector can
be used to determine the remaining utility of the contents.
In another embodiment of the invention the crystal is again placed in a
housing which has two input ports. Flexible tubes are attached to each of
the input ports and cap pieces, one or both of which may be mirrors, are
placed in the open ends of each of the flexible tubes. A desiccator or
other air treatment device is then attached to the system to remove
unwanted elements from the atmosphere inside the system and those that may
leak in over time. A light source is set up to direct light at the
contents of the desiccator or air treatment device. A detector can be set
up to receive light reflected from or light transmitted by the contents of
the desiccator. The amount of light received by the detector can be used
to determine the remaining utility of the contents.
In yet another embodiment of the present invention two of the faces of a
crystal are polished optically flat. An optical quality cap piece is then
optically contacted onto each of the polished faces of the crystal without
the use of any epoxy or other bonding agent. The cap pieces protect the
faces of the crystal form dust and moisture.
In another embodiment of the invention, each cap piece is positioned a
predetermined distance from a face of the crystal to form an etalon
between the face of the crystal and the inside face of the cap piece. A
barrier is then placed around a perimeter of the crystal face and the cap
piece to prevent unwanted environmental factors from entering the gap
between the crystal face and the inside surface of the cap piece.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is an illustration of a crystal isolation system that utilizes two
end caps to protect the faces of the crystal.
FIG. 2 is an illustration of a crystal isolation housing in which the two
end caps form etalons with the faces of the crystal.
FIG. 3 is an illustration of a crystal isolation housing in which the
crystal is placed inside a housing with two end caps.
FIG. 4 is an illustration of a crystal isolation housing in which the
crystal is placed inside a housing and the end caps are placed at the ends
of flexible tubes to allow the system to be more easily aligned to a laser
system.
DETAIL DESCRIPTION
Referring now to FIG. 1, environmental isolation housing 10 isolates
crystal 12 from the environment through the use of two cap pieces 14A and
B. Typically, cap pieces 14A and B are be made of a material which
transmit the wavelengths of interest and preferably has an index of
refraction approximately equal to the index of refraction of crystal 12 at
the wavelength of interest in order to help minimize reflections at the
interface. In one embodiment of the present invention, crystal 12 is LBO
and cap pieces 14 are made of F8 glass. The index of refraction in the
y-axis of LBO at 532 nm is 1.6065 and the index of refraction of F8 glass
at 532 nm is 1.6088. Thus the index differential between the two material
at this wavelength is only 0.0057. It will be appreciated by those skilled
in the art that if other types of glass may be used in order to meet
different optical, thermal or other material requirements.
Cap pieces 14 can be in any desired shape such as flat or lens-shaped as
long as they act as a barrier to the environmental factors of interest
such as moisture and dust. Additionally, they can be coated to reduce
reflection losses or can they can be partial reflectors or mirrors.
Inside surfaces 16A and B of cap pieces 14 A and B are optically contacted
with crystal faces 1 8A and B (respectively) to hold the two surfaces
together. One method that can be used to create the optically contacted
bond between crystal faces 18 and the cap pieces 14 is as follows. Crystal
faces 18 are polished optically flat and inside surfaces 16 are also
polished optically flat. Crystal face 18 and inside surface 16 of cap
piece 14 are carefully cleaned then pressed into contact for approximately
30 seconds. For surfaces that have atoms that form surface compounds
readily, cap pieces 14 will stay in place. The optical bonding procedure
can be found in published sources such as Prism and Lens Making by F.
Twyman (1952) pp. 241 and the references therein. This entire book and the
references therein are hereby incorporated by reference.
Crystal 12 may expand or contract in response to temperature changes due to
heaters, laser light, or the environment. Crystal 12 and cap pieces 14 may
have different thermal expansion coefficients. It may be difficult to find
a material for the cap pieces 14 which has both: (1) an index of
refraction that is close enough to that of crystal 12 such that the
reflections at the interface are tolerable and (2) a thermal expansion
coefficient close enough to that of crystal 12 such that cap pieces 14 do
not crack or separate during operation of the system. Cap pieces 14 can be
made thin enough such that they will expand without cracking as the
crystal expands. Thus, cap pieces 14 can be chosen such that an index of
refraction is substantially similar to an index of refraction of crystal
12 even if a thermal expansion coefficient of cap pieces 14 are different
from a thermal expansion coefficient of crystal 12.
For materials that do not readily form surface compounds, cap pieces 14 may
not stay in place on crystal faces 18. In this case, crystal 12 and cap
pieces 14 can be held against each other using, for example, springs 24.
Other devices for applying the necessary pressing forces over the
temperature range of interest can be used and will be readily apparent to
those skilled in the art.
The two cap pieces 14A and B act to protect crystal faces 18A and B from
undesirable environmental factors such as moisture or dust. Light can
enter crystal 12 through cap piece(s) 14 and crystal faces 18 will be
protected from environmental factors. Ordinarily, only polished crystal
faces 18 of crystal 12 need to be protected from environmental factors
such as dust and humidity because the rate at which moisture enters
crystal 12 through sides 19 is much slower than the rate at which moisture
enters crystal 12 through crystal faces 18. As a result, if only crystal
faces 18 are protected from moisture, crystal 12 is effectively protected
from deterioration due to moisture in the environment for a long period of
time.
Reflection losses from the outside surfaces 20A and B of transparent cap
pieces 14 can cause undesirable losses. To help reduce these losses,
outside surfaces 20 of transparent cap pieces 14 can have an
antireflection coating 22 put on them. Such coating are readily available
for most optical Classes but are not as available for crystals such as
LBO.
The portion of crystal 12 not covered by transparent end pieces 14 may be
covered with a coating or a material in order to isolate the rest of the
crystal from the environment. This may be desirable for certain types of
crystals that are highly hydroscopic such as sodium chloride.
Another embodiment of the present invention is shown in FIG. 2.
Environmental isolation housing 100 isolates crystal 102 from
environmental factors such as humidity and dust. Inside surface 106A of
cap piece 104A is held a predetermined distance from crystal face 108A
such that an etalon effect exists between inside surface 106A and crystal
face 108A. Inside surface 106B of cap piece 104B is held a predetermined
distance from crystal face 108B such that an etalon effect exists between
inside surface 106B and crystal face 108B. The passbands of these two
etalons may be selected to be any appropriate center wavelength and
bandwidth. Crystal 102 may be LBO and cap pieces 104 may be F2 glass.
Etalon spacers 110 can be shaped to environmentally isolate crystal faces
108A and B from the environment. For example, spacers 110 can be shaped
like a ring, an oval or any other closed shape which isolates crystal
faces 108 from the environment. Alternatively, environmental barriers 118A
and B can be placed around a perimeter of cap pieces 104 and crystal faces
108 in order to isolate crystal faces 108 from the environment.
Environmental barriers 118 can be made from any material that will prevent
the passage of deleterious environmental components such as dust and
moisture. Examples of suitable materials are some silicones and Teflon.
Etalon spacers 110 can be placed between each inside surface 106-crystal
face 108 pair. Etalon spacers 110 may be separate components which are
sized to hold inside surface 106 and crystal face 108 a predetermined
distance apart. Etalon spacers 110 can also be fabricated out of, attached
to, or deposited on inside surfaces 106 or crystal faces 108.
Retaining springs 112A and B may be used to apply pressure to cap pieces
104 so that there is physical contact between inside crystal surface 106A,
etalon spacer 110A and crystal face 108A, and between inside crystal
surface 106B, etalon spacer 1 OB and crystal face 108B.
The outside surfaces 114A and B of cap pieces 104A and B can also have an
antireflection coatings 116A and B coated on them. Reflection losses from
the outside surfaces 114A and B of cap pieces 104 can cause undesirable
losses. Antireflection coatings may be used to reduce these losses.
Additionally, as discussed above, the portion of crystal 102 not covered by
transparent end pieces environmental barriers 118 may be covered with a
coating or a material in order to isolate the rest of the crystal from the
environment.
Another embodiment of the invention is shown in FIG. 3. Environmental
isolation system 200 isolates crystal 202 from the environment. Housing
204 is closed to the environment except for two input ports 206A and B.
Cap piece 208A is placed over input port 206A and environmentally seals
input port 206A. Cap piece 208B is places over input port 206B and
environmentally seals input port 206B. Thus, when cap pieces 208 are
placed over input ports 206, housing 204 is environmentally isolated.
Crystal 202 is oriented within housing 204 such that crystal face 210A is
facing input port 206A and crystal face 210B is facing input port 206B.
One or both of the crystal faces 210A and B can be antireflection coated
to reduce losses due to reflections from the surfaces of the crystal.
Cap pieces 208 can be antireflection coated. A typical coating that may be
used in this regard is available from CVI Laser of Albuquerque, N. Mex.
Either or both of the inside surfaces 212A and B of cap pieces 208A and B
can be antireflection coated. Additionally, outside surfaces 214A and B of
cap pieces 208A and B can also be antireflection coated. Furthermore, it
would not depart from the present invention for one or both of cap pieces
208 to be a coated with a reflective coating. Also, one or both cap pieces
208 can be shaped as a lens or a mirror.
Inside faces 212 and outside faces 214 of cap pieces 208 can be slightly
angled to prevent etaloning effects with other surfaces. These surfaces
can be angled in a variety of ways, including angling the portion of the
housing to which the cap pieces are coupled, or wedging the faces of the
cap pieces 208. Inside faces 212 and outside faces 214 can also be
antireflection coated to reduce reflection losses or they can be coated to
act a full or partial reflectors.
Housing 204 can contain oven heater 216 for maintaining the crystal
substantially at a specified temperature. Additionally, a desiccator,
molecular sieve, or other filtering device 218 can be attached to housing
204. Due to manufacturing imperfections, material choices, design
oversights or other effects, housing 204 may not be completely isolated
from environmental factors such as moisture, dust or other undesirable
elements. Device 216 can be used to remove these unwanted factors that may
enter housing 204.
Another embodiment of the present invention is shown in FIG. 4.
Environmental isolation housing 300 isolates crystal 302 from undesired
environmental factors such as dust and moisture. Housing 304 is closed to
the environment except for two input ports 306A and B. Crystal 302 is held
in thin metal tube 308 and positioned such that crystal face 310A is
oriented towards input port 306A and crystal face 310B is oriented towards
input port 306B.
If heater 312 is desired to help temperature tune or temperature stabilize
crystal 302, then heater 312 can be placed on the outside of thin metal
tube 308. Heater 312 will heat crystal 302 through thin metal tube 308,
but because thin metal tube 308 is thin it will have poor thermal
conductivity and therefore heat will not be very effectively conducted to
other parts of system 300.
Thin metal passage 314A is inserted through input port 306A and into thin
metal tube 308. Thin metal tube 308 is held against thin metal passage
314A using adjustment screws 316. Thin metal tube 308 could also be sealed
to thin metal passage 314A using a canted coiled spring and seal. The BAL
SEALS seals manufactured by Bal Seal Engineering Company, Inc. can be
used. Thin metal passage 314B is also sealed to thin metal tube 308 using
set screws 316 but a canted coiled spring seal may also be used.
The other end of thin metal passage 3 14A is sealed to cap piece 318 using
a canted coiled spring and seal which can be integrated into mounting
block 320. Mounting block 320 can further be epoxyed to cap piece 318
and/or thin metal passage 314A. Cap piece 318 can be positioned with
respect to the rest of system 300 through the use of mount 322. Thin metal
passage 314A should be constructed to allow enough flexure so that cap
piece 318 can be aligned to system 300. Cap piece 318 can be any
appropriate optical component such as a lens or a mirror.
The end of thin metal passage 314B which is not sealed to thin metal tube
308 is sealed to mounting block 324 blocking one end of passage 326. Thin
metal passage 314B can be sealed to mounting block 324 using set screws, a
canted coiled spring seal or any other type of seal. The other end of
passage 326 is sealed with cap piece 328. Cap piece 328 can be sealed to
mounting block 324 using epoxy, a canted coiled spring seal or any other
type of seal. Cap piece 328 can be any appropriate optical component such
as a lens or a mirror. Thin metal passage 314B should allow enough
movement so that cap piece 328 can be aligned with system 300 by
manipulating mounting block 324.
Tube 330 is connected to thin metal passage 314A so that the atmosphere 332
contained in system 300 can be communicated down tube 330. Tube 330 could
be connected to any other part of the system which would allow access to
enclosed atmosphere 332. The other end of tube 330 is connected to
desiccator 334. desiccator 334 could be integrated into system 300 without
the use of a tube without departing from the present invention.
Desiccator 334 contains desiccant 336 which is used to remove moisture from
atmosphere 332. If there are other impurities in atmosphere 332 then other
scrubbing devices besides a desiccator can be used. For example, if it is
desired to remove dust from atmosphere 332 then desiccator 334 can be
replaced by an air filter.
Light source 338 emits light 340 towards desiccator 334. A portion of light
340 is reflected by desiccant 336 in desiccator 334. The reflected light
342 is detected by detector 344. Typically desiccant or an air filter will
change color as it is used. Thus, the spectrum and intensity of light
reflected from the desiccant will change as the desiccant is used up. For
example, when a red LED is used as the light source and DRIERITE type
desiccant manufactured by Hammond Drierite Company is used in the
desiccator, the intensity of the red light reflected from the desiccator
increases by a factor of 2 when the desiccant has been mostly used up.
Since the desiccant is typically used up starting from the portion of the
desiccator closest to the input tube, one or more LED-detector pairs can
be positioned at various points along the desiccator to provide desiccant
usage data or appropriate warnings as to how much desiccant has been used
up.
Environmental isolation housing 300 may be constructed with small
controlled leaks which will equalize the pressure inside and outside the
housing in order to avoid misalignment effects. Preferably, desiccator 334
can be constructed to hold enough desiccant 336 to provide long term
protection against moisture in the housing despite the small controlled
leaks.
The following descriptions are presented to enable any person skilled in
the art to make and use the present invention, and is provided in the
context of a particular application and its requirements. Various
modifications to the preferred embodiments will be readily apparent to
those skilled in the art, and the general principles defined herein may be
applied to other embodiments and applications without departing from the
spirit and scope of the present invention. Thus the present invention is
not intended to be limited to the embodiments shown, but is to be accorded
the widest scope consistent with the principles and features disclosed
herein.
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